The results of the ${}^{13}\mathrm{C}\text{NMR}$ spectra and nuclear spin-lattice relaxation rate $1∕T_1$ for the quasi-one-dimensional quarter-filled organic material ${\left(\text{TMTTF}\right)}_2\text{SCN}$ are presented. Below the anion ordering temperature $\left(T_{\text{AO}}\right)$, a new broad line appears in the NMR spectra and the intensity of the distinct line owing to the inner carbon site from the inversion center is almost halved. The remarkable difference in the temperature dependence of $1∕T_1$ below $T_{\text{AO}}$ for the two sharp lines corresponding to outer and inner carbon sites shows the development of a local electronic state. Our simple model of a charge configuration based on the electrostatic interaction between the SCN anions and TMTTF molecules is consistent with our observation of a local gap for the spin excitation. Nevertheless, we reveal that only the electrostatic interaction is insufficient to reproduce the observed divergence of the frequency shift and the linewidth of the newly appearing broad line stemming from the charge-accepting inner site at a much lower temperature than $T_{\text{AO}}$.

• Received 23 June 2003

DOI:https://doi.org/10.1103/PhysRevB.70.045102